Hindawi Publishing Corporation Journal of Nanomaterials Volume 2011, Article ID 642980, 9 pages doi:10.1155/2011/642980
Research Article Adding TiC Nanoparticles to Magnesium Alloy ZK60A for Strength/Ductility Enhancement Muralidharan Paramsothy,1 Jimmy Chan,2 Richard Kwok,2 and Manoj Gupta1 1 Department 2 Singapore
of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576 Technologies Kinetics Ltd. (ST Kinetics), 249 Jalan Boon Lay, Singapore 619523
Correspondence should be addressed to Manoj Gupta,
[email protected] Received 10 March 2011; Accepted 29 June 2011 Academic Editor: Theodorian Borca-Tasciuc Copyright © 2011 Muralidharan Paramsothy et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ZK60A nanocomposite containing TiC nanoparticles was fabricated using solidification processing followed by hot extrusion. The ZK60A nanocomposite exhibited similar grain size to monolithic ZK60A and significantly reduced presence of intermetallic phase, reasonable TiC nanoparticle distribution, nondominant (0 0 0 2) texture in the longitudinal direction, and 16% lower hardness than monolithic ZK60A. Compared to monolithic ZK60A (in tension), the ZK60A nanocomposite simultaneously exhibited higher 0.2% TYS, UTS, failure strain, and work of fracture (WOF) (+13%, +15%, +76%, and +106%, resp.). Also, compared to monolithic ZK60A (in compression), the ZK60A nanocomposite exhibited lower 0.2% CYS (−17%) and higher UCS, failure strain, and WOF (+11%, +29%, and +34%, resp.). The beneficial effect of adding TiC nanoparticles on the enhanced tensile and compressive response of ZK60A is investigated in this paper.
1. Introduction Magnesium alloys based on the Mg-Zn combination (such as ZK60A from the Mg-Zn-Zr system) are well known for their precipitation hardening characteristics during ageing [1]. Here, MgZn’ (or metastable β1 ’ phase) forms as rods parallel to the c-axis of the HCP unit cell, while metastable β2 ’ phase forms as discs parallel to the (0 0 0 2) basal plane of the HCP unit cell [2–6]. The β1 ’ phase has been reported to have a monoclinic structure similar to that of Mg4 Zn7 in aged Mg8 wt.% Zn alloy [2]. The yield strength of quasicrystalline particle reinforced Mg-Zn-Y and Mg-Zn-Y-Zr magnesium alloys was observed to increase with the volume fraction of the quasicrystalline phase [7]. This was based on the strengthening effect of the quasicrystalline particles [7]. Due to low particle-matrix interfacial energy, icosahedral particles in the Mg-Zn-Y alloy have been observed to be stable against coarsening during elevated temperature deformation [7]. ZK60A is commonly used in structural applications based on the following characteristics: (a) high strength and ductility after T5 aging, (b) good creep resistance, (c) poor arc weldability due to hot-shortness cracking, and (d) excellent resistance weldability. Al2 O3 nanoparticles were added to
ZK60A recently using disintegrated melt deposition (DMD) [8, 9] followed by hot extrusion and heat treatment. In the study, the Al2 O3 nanoparticles were agglomerated and not well dispersed in the ZK60A matrix [10]. This resulted in (1) tensile/compressive strength of ZK60A increasing (without significant loss in ductility) in the presence of 1.0 vol.% Al2 O3 nanoparticles and (2) tensile/compressive ductility increasing (without significant loss in strength) in the presence of 1.5 vol.% Al2 O3 nanoparticles [10]. It was also observed that Zr and Zn were leached out of the ZK60A matrix by the agglomerates of Al2 O3 nanoparticles [10]. Carbon nanotubes (CNTs) were also added to ZK60A recently using DMD [8, 9] followed by hot extrusion. Here, CNTs were not agglomerated but reasonably well distributed in the ZK60A matrix [11]. This enabled (1) simultaneous increase in tensile strength and ductility of ZK60A and (2) significant increase in compressive ductility (with significant decrease in compressive strength) of ZK60A [11]. Also, the intermetallic phase(s) precipitation was reported to be possibly regulated at nanoscale in this nanocomposite [11]. Dissolved Zn segregation at the liquid-SiC nanoparticle interface in a cast ZK60A/SiC nanocomposite which enabled nanoscale MgZn2 precipitation has also been discussed in detail [12].
2 Ti and C are not known to react actively with molten Mg. This is favourable concerning metal-matrix composite processing for structural applications where particlematrix interfacial reactions leading to inferior mechanical properties are undesirable. Regarding TiC reinforced metalmatrix composites, molten Al-Mg alloys were infiltrated at 900◦ C into TiC preforms with flowing argon [13]. Wetting of TiC substrates by the molten Al-Mg alloys was investigated in this work [13]. It was observed that (1) selective Al4 C3 formation was present at the matrix-preform interface and (2) TiAl3 traces were present in the Al-based matrix [13]. Also, a preform containing elemental powders of Ti and C (where TiC was formed in situ) was initially formed [14]. Molten magnesium alloy AZ91D was pressurelessly infiltrated into this Ti-C preform and tensile properties of the composite were compared to monolithic AZ91D [14]. Here, the in situ formed TiC reinforcement enhanced the tensile strength especially at higher temperatures [14]. The strain hardening exponent (n) of the AZ91D/TiC composite was higher at 0.71–0.82 compared to that of monolithic AZ91D being 0.11–0.32 (for tensile deformation carried out at 423–723 K in each case) [14]. The mechanical properties of hot pressed W were improved based on the addition of La2 O3 and TiC [15]. Here, the W matrix was strengthened due to the reinforcement particles pinning down the grain boundaries and inhibiting grain growth during sintering [15]. TiC particles exhibited good interfacial characteristics (for effective load transfer) with the adjacent W matrix while La2 O3 particles were beneficial for sintering and densification of composites [15]. The collective strengthening effect of La2 O3 and TiC particles on W was better than that of either La2 O3 or TiC [15]. It was recently reported that TiC formed in situ when CNT was added to pure Ti powder and the mixture was consolidated using spark plasma sintering followed by hot extrusion [16]. Here, the titanium matrix nanocomposite exhibited significantly higher yield and ultimate strengths without considerably compromising ductility compared to monolithic Ti [16]. Open literature search has revealed that no successful attempt has been made to simultaneously increase tensile strength and ductility of ZK60A magnesium alloy with TiC or any other Ti-based nanoparticles, using a high volume production spray-deposition-based solidification processing technique. Accordingly, one of the primary aims of this study was to simultaneously increase tensile strength and ductility of ZK60A magnesium alloy with TiC nanoparticles. Another aim of the present study was to evaluate the compressive properties of ZK60A/TiC magnesium alloy nanocomposite. Disintegrated melt deposition (DMD) [8, 9] followed by hot extrusion was used to synthesize the ZK60A/TiC magnesium alloy nanocomposite.
2. Experimental Procedures 2.1. Materials. In this study, ZK60A (nominally 4.80– 6.20 wt.% Zn, 0.45 wt.% Zr, balance Mg) supplied by Tokyo Magnesium Co. Ltd. (Yokohama, Japan) was used as the matrix material. ZK60A block was sectioned to
Journal of Nanomaterials smaller pieces. All oxide and scale surfaces were removed using machining. All surfaces were washed with ethanol after machining. TiC nanoparticles (98+% purity, 30– 50 nm size) supplied by Nanostructured and Amorphous Materials Inc. (Tex, USA) was used as the reinforcement phase. 2.2. Processing. Monolithic ZK60A was cast using the DMD method [8, 9]. This involved heating ZK60A blocks to 750◦ C in an inert Ar gas atmosphere in a graphite crucible using a resistance heating furnace. The crucible was equipped with an arrangement for bottom pouring. Upon reaching the superheat temperature, the molten slurry was stirred for 2.5 min at 460 rpm using a twin blade (pitch 45◦ ) mild steel impeller to facilitate the uniform distribution of heat. The impeller was coated with Zirtex 25 (86% ZrO2 , 8.8% Y2 O3 , 3.6% SiO2 , 1.2% K2 O and Na2 O, and 0.3% trace inorganics) to avoid iron contamination of the molten metal. The melt was then released through a 10 mm diameter orifice at the base of the crucible. The melt was disintegrated by two jets of argon gas oriented normal to the melt stream and located 265 mm from the melt pouring point. The argon gas flow rate was maintained at 25 dm3 min−1 . The disintegrated melt slurry was subsequently deposited onto a metallic substrate located 500 mm from the disintegration point. An ingot of 40 mm diameter was obtained following the deposition stage. To form the ZK60A/1.5 vol.% TiC nanocomposite (see Figure 1), TiC nanoparticle powder was isolated by wrapping in Al foil of minimal weight (
